The present invention relates to a low, open notch antenna, designated a Y-antenna, preferably intended as a mobile telephone antenna for motor vehicles, or alternatively for portable pocket telephones or personal pagers. The antenna comprises a Y-shaped sheet structure with two open notches quarter wave resonant, at different frequencies, electromagnetically coupled to one another and fed preferably by means of one feed unit at a common feed point. The antenna further includes a rectangular base plate of metal symmetrically mounted beneath and at right angles to the vertical shank of the Y, and galvanically connected thereto.
In certain cases when the antenna is placed on a large sheet metal area as is the case in private cars, the base plate can be secured to the sheet metal of the car by means of a retentive double-adhesive tape via which the antenna will moreover be capacitatively coupled to the subjacent sheet metal of the vehicle. In other cases when the antenna is mounted on a portable table pocket telephone or a personal pager, the base plate is designed so that it is possible, by filter effect, to obtain if necessary a relatively large coupling impedance between the antenna and the apparatus chassis.
Antennas for motor vehicles normally consist of a quarter wave long monopole antenna or a colinear Franklin antenna normally approx. three quarter waves high and, as far as portable telephones are concerned, the quarter wave or half wave long antennae are the most generally prevalent. Antennas with an open slot, so-called notch antennas (FIG. 1) are known in the art and described by Cary and Johnson.
However, it should be observed that, for example, a notch antenna (FIG. 1) described by Cary and Johnson does not give an omnidirectional radiation pattern, but a pattern of the cardioid type in the yz plane, which signifies maximum radiation in the direction of the positive x axis and zero radiation in the opposite direction. In this case, the notch antenna consists of a notch in a large sheet metal, as is apparent from FIG. 1.
If the sheet is cut considerably so that the remaining sheet around the notch, for example the distance between notch and edge, is of the order of magnitude of between 3 and 4 hundredths of a wavelength of that frequency where the open notch has its quarter wave resonance, and if this considerably cut notch sheet is placed at right angles above, and ideally in contact with, an earth plane sheet, there will be obtained an as good as omnidirectional antenna or, in other words, there will be obtained that antenna which is shown in FIG. 2 and whose radiation properties also substantially correspond to a quarter wave high monopole antenna when this, in a corresponding manner, is placed above an earth plane. The height of the notch antenna can be limited to less than a tenth of a wavelength (0.10 lambda) which is to be compared with the height of a monopole antenna which, at resonance, is just beneath a quarter wavelength (0.25 lambda).
The above described version of the notch antenna, which corresponds to the antenna embodiment according to FIG. 2, is known in the art. For many applications where an antenna of slight extent in the vertical direction and good omnidirectional radiation properties is desired, the above-described version of the notch antenna offers a satisfactory solution to the antenna problem as far as these properties are concerned, but it is a solution which, in many cases, can nevertheless not come into question because of the unsatisfactory band width properties of the antenna design and construction.